A large number of reasons exist for lighting a large underwater environment including security, safety and illumination of work surfaces. Applications include oil drilling platforms, lighting around submarines and ships and for storage pools. In all applications it is desirable to use a high-efficiency, long-lifetime light source which can provide continuous lighting with minimal maintenance. Nowhere is the need for a low maintenance lighting system more pronounced than in nuclear refueling pools, spent fuel storage pools and in nuclear reactor vessels. These structures contain water, which is used to limit the transmittal of radiation. Service of the lighting systems in these areas takes excessive time, personnel may have limited access, and their service results in exposure of the maintenance personnel to radiation.
Typically, these pools require a large number of lights for effective illumination. Traditionally, this lighting has been accomplished using 1000 W, 120 V incandescent spotlights or floodlights. These bulbs have lifetime ratings of 2,000 to 4,000 hours, and provide total light output of 17,000 lumens. At a lifetime of 4,000 hours, a particular light fixture will require 2.19 bulb changes per year, with maintenance personnel being exposed to radiation at each bulb change. A typical fuel storage pool uses 20 incandescent light fixtures. Thus, maintenance personnel may be subjected to short periods of radiation quite frequently for single bulb changes or to extended periods of exposure for “en mass” changes, if it is even possible to gain access to change the bulbs.
Inside a nuclear containment structure, water is normally contained only in the immediate area of the reactor itself, i.e., the reactor pressure vessel. However, when the reactor is shut down for a refueling outage, it is necessary to fill the entire refueling cavity with water, to limit the transmittal of radiation as the fuel is being unloaded and loaded. The reactor cavity is typically flooded only during this refueling outage period, but it is necessary during this time to make sure that the cavity is properly illuminated.
During this outage period, when maintenance is being performed on the reactor and when the fuel is being unloaded and loaded, it is costly and impractical to allocate maintenance personnel time for servicing the underwater lights. Additionally, some lamps may be installed in isolated areas where radiation flux can become quite high, such that access is available only for limited periods. The nuclear maintenance workers who are responsible for these areas are required to wear cumbersome PPE (Personal Protective Equipment) that makes high-dexterity repair work difficult or impossible. Every minute of radiation exposure is critical, excess radiation exposure is costly for plant owners, and personnel are limited in the cumulative amount of radiation exposure they can receive in a given time period. As a result of this challenging situation, in practice many of these short-lifespan lights remain failed rather than being continually serviced, often resulting in some of these critical structures being poorly illuminated. Even in areas where water is not introduced, a reliable, long-lasting light source is needed for replacement of the currently-used incandescent bulbs.
A number of underwater lights are the subjects of patents, however, for various reasons, these lights are not suitable for use in nuclear environments, either as fixed lights or as drop lights. The submersible light assemblies of Olsson et al. (U.S. Pat. No. 4,683,523, issued Jul. 28, 1987, and U.S. Pat. No. 4,996,635, issued Feb. 26, 1991) have funnel-shaped housings with flared front portions designed for fixed attachment to submersible vehicles. The light sources are quartz-halogen lamps which require heat sinks, and the lamps themselves are fully isolated from water. The housings are relatively large and cumbersome and not adjustable in direction once attached. The light produced is generally projected in a narrow beam forward from the lens. Such a construction would not be suitable for the wide angle illumination needed in a nuclear pool or for the maneuverability required for a cable-suspended drop light.
The underwater light of Poppenheimer (U.S. Pat. No. 4,574,337, issued Mar. 4, 1986) has a housing that is much larger than the small quartz-halogen lamp housed therein. The lamp is fully isolated from the water by an inner casing which is cooled by water that enters the outer housing. The light is projected forward in a generally narrow beam, resulting in the same limitations for use in nuclear applications as the lights of Olsson et al.
The high-intensity light source described by Mula (U.S. Pat. No. 5,016,151, issued May 14, 1991) has a watertight housing with a second subhousing to isolate the lamp from the water. The flared shape of the housing places limitations on the maneuverability of such a device as a drop light.
Finally, and most importantly, none of the above-described lights make provisions for rapid changeout of burned-out or damaged bulbs. The reliance on closed housing construction requires that any bulb changes be made out of the water, which is one of the main problems that must be overcome in a hazardous environment such as nuclear facility pools. Such changes are time-consuming and require multiple radiation exposures to effect a bulb replacement. Traditional incandescent underwater lamps used multiple small fasteners and sealing rings that necessitated a high level of dexterity for proper maintenance. If the entire lighting assembly were to be replaced to avoid multiple exposures, such changes could become very expensive due to the complex construction of the assemblies. Any facility which requires a large number of such light systems could find them to be prohibitively expensive to maintain
High pressure sodium (HPS) lighting has been used extensively for street and parking area illumination, lighting in factories and for security lighting. The primary advantages of HPS lights are: 1) high efficiency, and 2) very long lifetime. Compared to a 1000 W incandescent bulb, an HPS bulb has a lifetime rating of 24,000 hours and provides a total light output of 140,000 lumens. U.S. Pat. No. 5,105,346, No. 5,213,410 and No. 5,386,355, each incorporated herein by reference, describe a lighting system and method for lighting hazardous underwater environments using HPS lamps in a modular configuration that provides for rapid replacement of the damaged or burned-out bulbs. The commercial version of this lighting system has received universal acceptance from major nuclear fuel manufacturers and has been installed in a large number of nuclear power plants worldwide.
One drawback of HPS lighting is that its yellow-orange color temperature (˜2,200 K) is not ideal for human vision, which is optimized for white (5,500 K) light. While HPS lighting was the best option at the time the time of these patents, when using HPS lights in the underwater environment it can be difficult to discern objects and identify their true color due to the non-white color of the illumination. An additional drawback is that HPS lamps can take several minutes before reaching full intensity, which delays the user's ability to see clearly within the underwater environment in an emergency situation, if these lights were not previously turned on.
The recent emergence of ultra-bright, white, high power light emitting diodes (LEDs) presents an alternative that can overcome some of the above-described drawbacks of HPS lighting. Key characteristics of these high power LEDs are excellent reliability and durability, instant turn on, longevity and good color. Furthermore, the efficiency (increased lumens per watt) of these LEDs provides a significant reduction in power consumption and, consequently, carbon emission. However, complexities are introduced over traditional lighting sources by their need for drivers and power factor correction. Despite these advantages, the major issue that has previously prevented the adoption of LED lighting is thermal management. While typical LEDs can be operated at temperatures up to 185° C., that high of an operating temperature is not conducive to long life and low maintenance. Some LED manufacturers specify a maximum operating temperature of 85° C. to ensure 70% luminance after an operating life of 50,000 hours. Failure to address the heat dissipation needs of LED lighting will lead to severe degradation, which reduces operational lifetime, reduces visible light output, and negatively affects the color rendering.
U.S. Pat. No. 6,412,971 describes a LED array that has a large number of elements arranged with sufficient density to achieve a desired illumination intensity to replace conventional incandescent or HPS light sources without creating the environmental concerns of fluorescent bulbs. While the disclosed LED array solves many of the problems encountered with replacement of incandescent bulbs with LED arrays, it does not provide solutions for the special requirements of underwater operation, and particularly fails to address the problems involved in underwater operation in a hazardous environment such as a nuclear spent fuel pool or nuclear reactor.
Accordingly, obstacles remain to realization of LED-based lighting fixtures for use in hazardous underwater environments such as nuclear reactors and spent fuel pools. The present invention is directed to providing such fixtures.